Process for the separation of aromatic hydrocarbons from a mixed hydrocarbon feedstock

ABSTRACT

The process involves a combination of continuous solvent extraction-steam distillation for the recovery of aromatic hydrocarbons from a mixed feedstock. The feedstock is contacted with a solvent-water mixture at temperatures in the range of about 75* to 200* C and the extract and raffinate streams are separated into their components. The purity of the aromatics recovered from the extract is improved by introducing light aliphatic hydrocarbons consisting essentially of aliphatic and cycloaliphatic hydrocarbons of no more than five carbon atoms with the feedstock at the middle theoretical stage of the extraction column. The light aliphatics are introduced in amounts in the range of 5 to 12 percent, based on the weight of the feedstock.

United States Patent [1 1 Kubek et a1.

[ Dec. 18, 1973 [75] Inventors: Daniel John Kubek, North Tarrytown;Alexander Jean-Marie Kosseim, Yorktown Heights; George Solomon Somekh,New Rochelle, all of NY.

[73] Assignee: Union Carbide Corporation, New

York, NY.

[22] Filed: Dec. 21, 1971 [21] Appl. No.: 210,404

3,470,088 9/1969 Vickers 260/674 2,840,620 6/1958 Gcrhold et a1 r260/674 3,642,614 2/1972 VanTassell 208/321 3,179,708 4/1965 Penisten203/321 3,436,435 4/1969 VanTassell.... 260/674 3,173,966 3/1965 Joneset a1, 208/321 Primary Examiner-Delbert E. Gantz Assistant ExaminerC. E.Spresser AttorneyPaul A. Rose et al.

[57] ABSTRACT The process involves a combination of continuous solventextraction-steam distillation for the recovery of aromatic hydrocarbonsfrom a mixed feedstock. The feedstock is contacted with a solvent-watermixture at temperatures in the range of about 75 to 200C and the extractand raft'mate streams are separated into their components. The purity ofthe aromatics recovered from the extract is improved by introducinglight aliphatic hydrocarbons consisting essentially of aliphatic andcycloaliphatic hydrocarbons of no more than five carbon atoms with thefeedstock at the middle theoretical stage of the extraction column. Thelight aliphatics are introduced in amounts in the range of 5 to 12percent, based on the weight of the feedstock.

16 Claims, 1 Drawing Figure /6 25 29 24 a tag:

PROCESS FOR THE SEPARATION OF AROMATIC HYDROCARBONS FROM A MIXEDHYDROCARBON FEEDSTOCK FIELD OF THE INVENTION This invention relates toan improvement in a process for the separation of aromatic hydrocarbonsfrom a fixed hydrocarbon feedstock and, more particularly, to therecovery of high purity aromatic hydrocarbons in high yields whilemaking efficient use of process components. I

DESCRIPTION OF THE PRIOR ART With the advent of the benzene-toluene-Caromatics fraction (known and hereinafter referred to as BTX) as theprincipal raW rnateri'al in the manufacture of petrochemicals,outstripping ethylene in this regard, and the increased demand foraromatics as a component in gasoline to increase its octane rating andthus reduce or eliminate the need for lead, which has been under fire asa pollutant, aromatics separation processes availed of in the past havecome under close scrutinywith an eye toward improving process economics.

Improved economics can be translated into, among other things, thelowering of heating requirements and the more effective use of processcomponents as aids in the separation process.

Various processes have been used for aromatics separations in systems ofthe single extractor-single distillation column (or stripper) type,which have one particular step, among others, in common, i.e., thereflux hydrocarbons are derived from the distillation of the extract andare recycled into the lower portion of the extraction column below thebottom plate or below the lowest theoretical stage. To improve thepurity of the aromatic products, lower boiling aliphatics, not preciselythe light aliphatics defined below, have been added to this reflux.These lower boiling aliphatics were obtained from fractionation of thefeedstock, raffinate, and/or reflux and always introduced with or as apart of the reflux at the bottom of the extractor.

While purities have been improved and heating requirements lowered usingthis technique, optimization has not been achieved.

SUMMARY OF THE INVENTION An object of this invention, therefore, is toimprove the reflux in aqueous solvent single extractor-singledistillation column systems whereby heating requirements are reduced andthe purity of the aromatics is raised to previously unattained levels.

Other objects and advantages will become apparent hereinafter.

According to the present invention, high purity aromatic hydrocarbonsare effectively recovered using minimal heat in a continuous solventextractiondistillation process for the recovery of aromatic hydrocarbonshaving boiling points in the range of about 80C. to about 175C. from afeedstock containing aliphatic hydrocarbons and at least about 40percent by weight, based on the weight of the feedstock, of aromatichydrocarbons wherein the feedstock contains no more than about 4 percentby weight, based on the weight of the feedstock, of light aliphaticsconsisting essentially of aliphatic and cycloaliphatichydrocarbons,-each having no more than 5 carbon atoms, a boiling pointno higher than about 50C., and being condensable at a pressure of nomore than about 3 atmospheres and at a temperature of no less than about50C.; a single extraction column having about 3 to about 25 theoreticalstages is used for the solvent extraction to provide an extract; and asingle distillation column is used to distill the extract to provide amixture of the aromatic hydrocarbons comprising the following steps:

a. introducing the feedstock into the extraction column at the middletheoretical stage thereof;

b. contacting the feedstock in the extraction column with a mixture ofwater and a solvent, said solvent being a water-miscible organic liquidhaving a boiling point of at least about 200C. and having adecomposition temperature of at least about 225C, and with refluxhydrocarbons introduced into the extraction column below the bottomtheoretical stage thereof to provide an extract comprising aromatichydrocarbons, reflux aliphatic hydrocarbons, solvent, and water and araffinate comprising essentially aliphatic hydrocarbons;

c. introducing the extract into the distillation column to separate thearomatic hydrocarbons and the reflux hydrocarbons from the extract;

(1. recycling the reflux hydrocarbons from step (c) to the extractioncolumn as provided in step (b); and

e. recovering the aromatic hydrocarbons of step (d);

the improvement comprising introducing a sufficient amount of the lightaliphatics defined above into the extraction column at the middletheoretical stage thereof to provide a total percent by weight of saidlight aliphatics, based on the weight of the feedstock, in the range ofabout 5 percent to about 12 percent.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a schematic flowdiagram of an illustrative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As noted, there is an industrialneed for BTX, which is available in high proportion,e.g., at least about40 percent by weight, in a wide variety of hydrocarbon feedstocks suchas reformed gasolines; coke oven light oils; cracked gasolines; anddripolenes, which, after hydrogenation, can contain as much as to 98percent BTX. These feedstocks also contain both aliphatic andcycloaliphatic hydrocarbons (herein sometimes referred to collectivelyas aliphatic hydrocarbons). Since the individual hydrocarbon compoundswhich make up these feedstocks are well known, they will not bediscussed extensively; however, it can be pointed out that the majorcomponents of the feedstocks used herein are hydrocarbons with boilingpoints ranging from 25C. to C. including straight-chain andbranchedchain paraffms and naphthenes, such as n-heptane, isooctane, andmethyl cyclohexane, and aromatics such as BTX.

The BTX fraction can include benzene, toluene, the C aromatics includingortho-xylene, meta-xylene, para-xylene, and ethyl benzene, and Caromatics, which, if present at all, usually appear in the smallestproportion in relation to the other components.

It is also important to point out that conventional feedstocks used inextraction-distillation systems of the type discussed herein for therecovery of BTX contain about 1 to 3 percent and, in some cases, up toabout 4 percent by weight based on the weight of the feedstock, of lightaliphatics which are defined herein as aliphatic and cycloaliphatichydrocarbons having no more than 5 carbon atoms in each compound, saidcompound having a boiling point no higher than about 50C. and beingcondensable at a pressure of no more than 3 atmospheres and atemperature of no less than 50C. (hereinafter referred to as the definedlight aliphatics).

These defined light aliphatics provide the crux of subject improvementsince it has been found that when at a particular percentage level andwhen introduced at a particular point in the extraction column, theyenhance the reflux to a point where there is a noticeable andadvantageous drop in heating requirement together with a higher level ofpurity.

The use of this improvement is found to be advantageous in any processfalling within the process definition set out heretofore under thesummary of the invention; however, it is found to be particularlyadvantageous when applied to the preferred embodiment describedhereinafter.

The process described here, exclusive of subject improvement, is thesubject of our copending application Ser. No. 180,996, filed on Sept.16, 1971, now US. Pat. No. 3,714,033. This application bears the sametitle as the instant application and is incorporated by referenceherein.

A typical breakdown of the defined light aliphatic fraction, in percentby weight based on the total weight of the fraction, is about percent toabout 95 percent of the full range of C aliphatics, about 0 percent toabout 5 percent of C aliphatics, and a small proportion, about 0 percentto about 1 percent, of C to C aliphatics.

The solvents used in the subject process are, as described above,water-miscible organic liquids, (at process temperatures) having aboiling point of at least about 200C. and having a decompositiontemperature of at least about 225C. The term water-miscible includesthose solvents which are completely miscible over a wide range oftemperatures and those solvents which have a high partial miscibility atroom temperature since the latter are usually completely miscible atprocess temperatures. The solvents are also polar and are generallycomprised of carbon, hydrogen and oxygen with some exceptions. Examplesof solvents which may be used in the process of this invention aredipro' pylene glycol, tripropylene glycol, dibutylene glycol,tributylene glycol, ethylene glycol, diethylene glycol, ethylene glycolmonomethyl ether, ethylene glycol monoethyl ether, diethylene glycolmonomethyl ether, diethylene glycol monoethyl ether, sulfolane, N-methylpyrrolidone, triethylene glycol, tetraethylene glycol, ethylene glycoldiethyl ether, propylene glycol monoethyl ether, pentaethylene glycol,hexaethylene glycol, and mixtures thereof. The preferred group ofsolvents is the polyalkylene glycols and the preferred solvent istetraethylene glycol.

Additional solvents, which may be used alone or together, or with theaforementioned solvents are amides such as formamide, acetamide,dimethylformamide, diethylformamide, and dimethylacetamide;- amines suchas diethylenetriamine and triethylenetetramine; alkanolamines such asmonoethanolamine, diethanol-- amine, and triethanolamine; nitriles suchas beta,betaoxydipropionitrile and beta,beta -thiodipropionitrile;

phenol and the cresols; the methyl sulfolanes; sulfoxides such asdimethyl sulfoxide and diethyl sulfoxides; lactones such asgamma-propiolactone and gammabutyrolactone.

The apparatus used in the process both for the main extraction anddistillation is conventional, e.g., an extraction column of themultistage reciprocating type containing a plurality of perforatedplates centrally mounted on a vertical shaft driven by a motor in anoscillatory manner can be used as well as columns containing pumps withsettling zones, sieve trays with upcomers, or even a hollow tube whilethe distillation can be conducted in a packed or bubble platefractionating column. Countercurrent flows are generally utilized inboth extraction and distillation columns.

Heat exchangers, decanters, reservoir, solvent regenerator, andraffinate still are also conventional as well as various extractorsother than the main extractor. These other extractors are preferablysingle stage mixer-settlers, but can be any of the well known types.

The number of theoretical stages in the extraction column can be aboutthree to about 25 stages and is preferably about five to about 12stages. The middle theoretical stage of the column is defined to includethe theoretical stages running from 0.25 times the theoretical stages inthe column plus one to 0.75 times the theoretical stages in the samecolumn, e.g., in a column having four theoretical stages, the middletheoretical stage would include, as defined herein, the second and thirdtheoretical stages and in a column having 24 theoretical stages, themiddle theoretical stage would include the seventh to the 18ththeoretical stages, inclusive. It will be recognized by those skilled inthe art that the number of theoretical stages is to be considered asbased on functioning theoretical stages and that the bottom theoreticalstage is located just above the reflux inlet no matter what the actualphysical layout of the column is. The feed and the defined lightaliphatics can be introduced anywhere in the middle theoretical stage;however, it is preferred that both be introduced at the same point inthe middle theoretical stage to obtain the full benefits of the process.

The solvent is used as an aqueous solution thereof containing water inan amount of about 1 percent to about 8 percent by weight based on theweight of the solvent and preferably containing water in an amount ofabout 2 percent to about 5 percent by weight. This aqueous solution isreferred to hereafter in some instances as a solvent-water mixture.

Generally, to accomplish the extraction, the ratio of solvent (exclusiveof water) to feedstock in the extractor is in the range of about 4 toabout 8 parts by weight of solvent to one part by weight of feedstock.This broad range can be expanded upon where nonpreferred solvents areused. A broad range of about 3 to about 12 parts by weight of solvent toone part by weight of feedstock and a preferred range of about 5 partsto about 7 parts of solvent per part of feedstock can be usedsuccessfully for the solvent of preference and other like solvents. Infinal analysis, however, the ratio is selected by the technician basedon experience with the particular feedstock and depends in part uponwhether high recovery or high purity is being emphasized, although theinstant process will improve purity in any case.

The reflux to the extraction zone is generally made up of about 20percent to about percent by weight aliphatics having from five to sevencarbon atoms and about 50 percent to about 80 percent by weightaromatics, both based on the total weight of the reflux. The reflux,percentages, and ratios discussed in this paragraph are not consideredto include and do not apply to the defined light aliphatics, althoughthe latter might be referred to as an auxiliary reflux. There is someoverlap in composition, however, in that some of the components whichmake up the reflux are the same as some of the defined light aliphatics.These similar components in the reflux should not be included whendetermining whether the percentage of defined light aliphatics meets thestated requirements. In any event, the sources, other than thefeedstock, are different for bothin that the major source of the refluxis the distillate from the main distillation column while the source ofthe defined light aliphatics is from raffinate distillation or fromoutside the system. The ratio of reflux to feed-stock in the extractionzone is, generally, maintained in the range of about 0.5 to about 1.5parts by weight of reflux to one part by weight of feedstock andpreferably about 0.6 to about 1.25 parts by weight of reflux to one partby weight of feedstock, but, again, is selected by the technician justas the ratio of solvent to feedstock. The reflux aliphatics pass intothe extract rather than being taken overhead with the raffinate and arerecycled to the extractor from the reflux decanter as will be seenhereinafter.

The temperature in the extraction zone is maintained in the range ofabout 100C. to about 200C. and is preferably in the range of about 125C.to about 150C, especially for the solvent of preference.

The pressure in the extraction zone is maintained in the range of about75 psig. to about 200 psig. As is well known in the art, however, oneselected pressure is not maintained throughout the extraction zone, but,rather, a high pressure within the stated range is present at the bottomof the zone and a low pressure again within the stated range is presentat the top of the zone with an intermediate pressure in the middle ofthe zone. The pressures in the zone depend on the design of theequipment and the temperature, both of which are adjusted to maintainthe pressure within the stated range.

The temperature at the top of the distillation zone, which, in terms ofthe apparatus used, may be referred to as a distillation column orstripper, is at the boiling point of the mixture of aromatics present inthe zone while the temperature at the bottom of the stripper isgenerally in the range of about 135C. to about 200C.

The pressure at the top of the stripper, an upper flash zone in thiscase, is in the range of about 20 psig to about 35 psig. In a lowerflash zone just beneath the upper flash zone and connected thereto, thepressure is in the range of about psig to about 20 psig and is about 10or psig lower than the pressure in the upper flash zone. The pressure inthe rest of the distillation zone is maintained in the range of about 15psig to about 25 psig with some variation throughout the zone.

The steam brought into the bottom of the distillation zone enters at atemperature of about 100C. to about 150C. and is under a pressure ofabout 15 psig to about 25 psig. The total water present in thedistillation column is essentially in vapor form and is generally in therange of about 0.1 parts to about 0.5 parts by weight of water to onepart by weight of aromatics in the zone and preferably in the range ofabout 0.1 parts to about 0.3 parts by weight of water to one part byweight of aromatics. The water used for the steam may be calledstripping water. A small amount of water is present in liquid form inthe distillation zone dissolved in the solvent.

Referring to the drawing:

The feedstock and defined light aliphatics are introduced through line 1into heat exchanger 2 where they are preheated to a temperature in therange of about 50C. to about 150C. The mixture then continues throughline 1 to enter extractor 3 at the middle tray thereof, which is theequivalent of the middle theoretical stage. An aqueous solvent solutionhaving a temperature in the range of about 125C. to about 175C. entersat the top tray of extractor 3 through line 4 and percolates down thecolumn removing aromatics from the feedstock.

The raffinate, essentially free of aromatics, leaves the top of thecolumn and passes through heat exchanger 2 where it is used to preheatthe feedstock and is cooled in tum to a temperature in the range of C.to about 125C. The raffinate comprises about percent to about 98 percentby weight aliphatics, about 1 percent to about 3 percent by weightdissolved and entrained solvent, and about 0 percent'to about 3 percentby weight aromatics. The raffinate then passes through cooler 6 where itis further cooled to about 25C. to about 50C. and proceeds along line 5to decanter 7 where it separates into two phases, an aliphatichydrocarbons phase and a solvent phase, the solvent being contaminatedwith aliphatics.

It should be noted that the phase" is named after its main component,which is present in the phase in an amount of at least 50 percent byweight and, in most cases, in an amount of at least 90 percent byweight.

The aliphatic hydrocarbons phase, which can still be referred to as theraffinate, now contains about 96 percent to about 99 percent by weightaliphatics, about 0 percent to about 1 percent by weight dissolved andentrained solvent, and about 0 percent to about 3 percent by weightaromatics. The solvent phase, on the other hand, contains about 90percent to about 96 percent by weight solvent, about 2 percent to about5 percent by weight water, and about 2 percent to about 4 percent byweight aliphatics.

The raffinate continues overhead through line 5 into raffinate extractor8, which can be a single stage mixersettler or other conventional typeof extractor.

The solvent phase passes through line 10 to join line 12 referred tohereinafter or it can be optionally recycled to the top of extractor 3along line 20 (connection to extractor not shown).

The raffinate is washed with a portion of the water phase from refluxdecanter 29 and separated in raffinate extractor 8 into an aliphatichydrocarbons phase (still called the raffinate) which is essentiallyfree of solvent and water and contains about 97 percent to about 100percent by weight aliphatics and about 0 percent to about 3 percent byweight aromatics, and a raffinate water phase as bottoms which containsabout 75 percent to about 90 percent by weight water, about 10 percentto about 25 percent by weight solvent, and about 0.1 percent to about 1percent by weight aliphat- The raffinate proceeds along line 55 toraffinate still 56 where the defined light aliphatics are removed as avapor overhead and pass through condenser 58. A portion of thecondensate is returned to the top tray of raffinate still 56 as a refluxwhich aids in purifying the lights by knocking down the high boilers.The heavier aliphatics are removed as bottoms and pass through line 62where a portion is diverted through reboiler 63 and returns to raffinatestill 56 below the bottom tray as a vapor to provide most of the heatingrequirement. The balance of the heavy aliphatics proceeds along line 62to storage (not shown).

The balance of the condensate proceeds along line 60 where it joins line1 thus supplying a high proportion of the defined light aliphatics forthe process. This raffinate distillation is the preferred and mostsatisfactory mode for supplying the bulk of the defined light aliphaticsfor the process. About 50 percent to about lOO percent by weight of thedefined light aliphatics can be supplied in this manner althoughinitially the necessary defined light aliphatics must be provided froman outside source (not shown) to line 1 and make-up defined lightaliphatics may be added from time to time to maintain the requiredlevel. The raffinate still is similar to stripper 23 except that it canbe half the height and half the diameter thus reducing its cost andheating requirements. The temperature in the bottom of the raffinatestill is maintained in the range of about 45 C. to about 200 C., thepressure is maintained in the range of about 5 psig to about 40 psig,and the reflux ratio is about 0.5:l to about 3.0:].

Part of the raffinate water phase can optionally be recirculated throughextractor 8 via line 11, line 9 and line 5 as shown. This recirculationis conventional with a mixer-settler arrangement, but may not beadvantageous with other types of extractors. As noted, this water phasestill contains, along with the water and solvent, a small amountofaliphatics. All of the balance of the water phase is, therefore,directed from line 11 along line 12 to extractor 13, which can again bea single-stage mixer-settler.

Feeding into line 12 via line 50 is an aromatics slipstream, which atits source (see line 14) is an essentially pure stream of aromatics,i.e., having a purity of at least 95 percent by weight, or in otherwords, at least 95 percent by weight of the slipstream is aromatichydrocarbons. The purity of the slipstream is preferably about 98percent and for optimum performance, i.e., to obtain the highest purityproduct, about 99 percent or even higher. It is called a slipstream orsidestream because the amount of aromatics fed into the water phasepassing through line 12 is very small. The amount of slipstream aromatichydrocarbons used. in the process is in the range of about 0.1 percentto about 5 percent by weight of the aromatic hydrocarbons in thefeedstock and is preferably in the range of about 0.5 percent to about2.0 percent by weight of such aromatic hydrocarbons. The slipstreamwashes the water in extractor 13 to remove the small amount ofaliphatics, which is so detrimental to the efficiency of the process.This aromatics slipstream can be recycled along line 15 throughextractor 13 to further wash the water phase where a mixer settlerextractor is used and it is then, preferably, sent along line 16 to line1 where it is reintroduced into the feedstock and passes into the systemonce more. The water, which is essentially devoid of aliphatics, butcontains solvent, then passes as bottoms from extractor 13 through line17 and into water reservoir 51 via line 37.

Returning to extractor 3, it has been noted above that the aqueoussolvent percolates down the column carrying with it the aromatics. Inthe middle of the column, the aqueous solvent comes into contact withboth the feedstock and the defined light aliphatics. Although themechanism involved is not clear it is believed that the defined lightaliphatics initially build up in the extract until a saturation point isreached and then they pass into the raffinate. The effect of the definedlight aliphatics once the saturation point is reached is to reduce theamount of aliphatics which pass into the extract or, in other words, toimprove the selectivity of the solvent solution and enhance the activityof the reflux. In the lower half of extractor 3, the solvent solution ofaromatics comes into countercurrent contact with a reflux liquid, whichenters extractor 3 below the bottom tray (or theoretical stage) alongline 18. The reflux percolates up the lower half of extractor 3progressively dissolving in and purifying the solvent solution ofaromatics. The solution which is formed, i.e., the extract, comprisesabout 10 percent toabout 20 percent by weight feedstock aromatics, about2 percent to about 5 percent by weight water, about 65 percent to aboutpercent by weight solvent, about 4 percent to about 8 percent by weightreflux aromatics, and about 3 percent to about 6 percent by weightreflux aliphatics, all based on the total weight of the extract.

The extract leaves the bottom of extractor 3 through line 19 and passesthrough heat exchanger 22 where it is cooled to a temperature in therange of about 100C. to about 125C. The extract proceeds along line 19and enters stripper 23, the distillation zone, at upper flash chamber24, which, as noted heretofore, is at a lower pressure than theextractor. Part of the extract flashes on entering the flash chamber andis taken overhead through line 18 in vapor form. Another part of theextract passes as a liquid into lower flash chamber 21, which isoperated at an even lower pressure and further flashing occurs. Theseflashed vapors join the fractionated vapors and pass through line 30 tojoin the vapors passing through line 18. The balance of the extract (atleast about percent by weight) percolates down the column into thefractionation zone where it comes into countercurrent contact with thestripping vapors, i.e., steam, and more vapors are generated. A part ofthe vapor rises to the top of the column and mixes with the flashedvapors in flash chamber 21 as noted. The overhead distillate comprisesabout 40 to about 75 percent by weight aromatics, about 20 to about 40percent aliphatics, about 2 percent to about 10 percent by weight water,and about 0 percent to about 5 percent by weight solvent, all based onthe total weight of the overhead distillate.

After the aqueous solvent descends about halfway down the column, itbecomes essentially free of aliphatics. At this point, a vaporside-stream distillate is removed through line 26. The side-streamdistillate is comprised of about 65 to about percent by weightaromatics, about 10 to about 30 percent by weight water, and about 1percent to about 10 percent by weight of solvent, based on the totalweight of the side-stream distillate.

The bulk of the solvent and water solution, an amount equal to over 98percent by weight of the solvent and water entering stripper 23 throughline 19, leaves the bottom of stripper 23 through line 4. A portion ofthis solution is diverted into reboiler 28 and returns as a vapor to apoint below the bottom tray of stripper 23 to provide most of thestrippers heating requirements. The balance of the water and solventextraction is recycled to the top tray of extractor 3 through line 4.Recycled stripping water containing some dissolved solvent entersstripper 23 through line 27 from water reservoir 51 after essentiallyall of it is converted in heat exchanger 22 to steam.

Returning to the overhead distillate mentioned heretofore, such overheaddistillate is a combination of flashed vapors and fractionated vaporshaving the aforementioned composition. This overhead distillate is alsoknown as a reflux distillate. The vapor is first condensed and cooled tobetween about 38C. and 94C. in reflux condenser 25. The condensate thenpasses into reflux decanter 29 where a reflux hydrocarbons phase isdecanted from a water phase. The reflux hydrocarbons phase comprisesabout 20 to 50 percent by weight aliphatics having from five to sevencarbon atoms, and about 50 to about 80 percent by weight aromatics andis recycled as reflux through line 18 to extractor 3 as previouslydescribed.

The water phase contains about 95 to about 99 percent by weight water,about to about percent by weight solvent, and about 0.1 to about 0.5percent by weight aliphatics. It passes through line 31 and is split intwo streams, lines 32 and 33, a rafflnate wash stream and an aromaticswash stream, respectively. These washes can take place as shown bysplitting the stream or the entire stream can be used to wash therafflnate first and then the aromatics providing that the water istreated with an aromatics slipstream before the aromatics wash.

As noted heretofore, the side-stream distillate is withdrawn in vaporform from stripper 23 through line 26 and condensed in aromaticscondenser 34 and further cooled to a temperature in the range of about25C. to about 50C. in cooler 35, which can be a heat exchanger or othertype of cooling device. The condensate then passes into aromaticsdecanter 36 where an aromatic hydrocarbons phase containing about 99.8to about 99.9 percent by weight aromatics, and about 0.1 to about 0.2percent by weight solvent and a water phase containing about 90 percentto about 98 percent by weight water, about 2 percent to about percent byweight solvent, and about 0.1 percent to about 0.5 percent by weightaromatics are formed. The water phase passes through line 37 to waterreservoir 51. Optionally, all of part of the water phase can be directedthrough valved line 38 to join line 32 for use as rafflnate wash.

The aromatic hydrocarbons phase proceeds from decanter 36 through line26 along which an aromatics slipstream is taken through line 14 to washwater coming from reflux decanter 29 along line 33. As noted, thisslipstream can be in the range of about 0.10 percent to about 5.0percent of the total aromatics in the feedstock and is preferably in therange of about 0.50 percent to about 2.0 percent of the total aromaticsin the feedstock. These percentages are by weight.

In practice, the weight of the total aromatics is deter mined byanalysis of a sample portion of the feedstock. Aromatics added, e. g.,as slipstream, during the process cycle are included in thedetermination.

The slipstream can, alternatively, be obtained from another source suchas the overhead product of a benzene fractionating column, which is notshown in the drawing, or from a source completely removed from thesystem. Aslong as the slipstream has the previously noted high aromaticscontent, it will be satisfactory in this process.

The combined streams of lines 33 and 14 proceed into wash extractor 39,which can be a single stage mixer-settler or other. form of extractor.Where a mixersettler is used, it is advantageous to use an aromaticsrecycle which passes along line 42 and joins lines 33 and 14 returningto wash extractor 39. The slipstream, now containing a small amount ofaliphatics, passes overhead from wash extractor 39 into line 42 andalong line 50 to join lines 12 and 15 and proceeds into wash extractor13 as discussed previously.

Reflux water, now essentially free of aliphatics, is withdrawn from washextractor 39 and proceeds along line 43, which joins line 26, and passesinto aromatics extractor 44, which can be a single stage mixer-settleror other type of extractor. This reflux water, along with water recycledfrom the settling zone in the case of a mixer-settler via line 45, whichjoins line 43, and process makeup water from line 46 (source not shown)contacts the aromatic product proceeding along line 26 into aromaticsextractor 44 and recovers essentially all of the small amount of solventremaining in the aromatics. This water with solvent then proceeds alongline 47 to join line 17, which joins line 37 and enters water reservoir51. High purity aromatic product is withdrawn from the process throughline 26.

Removal of certain impurities, which may include some aliphatics of atype which can build up in the system and affect it in a deleteriousmanner, is accomplished by taking a small purge of the water circuit. Toaccomplish this purge, water is withdrawn from any of the decanters anddiscarded periodically or continuously. One such purge can beaccomplished through line 48. It is found that only a small proportionof the solvent is lost by such a purge; however, this solvent can berecovered if desired. The water purge stream can be in the range ofabout 0.25 percent to about 2.0 percent by weight of the total water inthe system and is preferably in the range of about 0.5 percent to about1.0 percent by weight of the water in the system.

The total water in the system can be determined easily because theamount of water introduced can be controlled. Allowances must be madefor water losses through leakage, entrainment and upsets, however.

Solvent can be recovered from this purge by directing the water throughline 49 to join line 53 and enter solvent regenerator 52 where thesolvent is separated from low boiling and high boiling impurities bysteam distillation under vacuum. The solvent is recovered and recycledalong line 54 to extractor 3 (connection not shown) and the water andimpurities discarded.

It will be noted that in the preferred embodiment of subject process theslipstream taken through line 14 is first used to wash the water phasefrom reflux decanter 29 (i.e., one stream) and then the water phasesfrom rafflnate decanter 7 (optional) and rafflnate extractor 8. Thisprocedure can be varied so that a different slipstream from a differentsource is used for each wash or, as previously mentioned, a singleslipstream is used to wash one water phase where stream 31 is not split,but is first used to wash rafflnate.

In the preferred embodiment, it was stated heretofore that theslipstream picks up some aliphatics in extractor 39 before proceeding toextractor 13. It should be pointed out that the purity of thisslipstream containing the small amount of aliphatics is only reduced byabout one percent and that it still has a purity of at least about 95percent by weight and preferably about 98 percent so that the definitionof the slipstream with respect to purity is fulfilled.

The description of the invention is in terms of a continuous processwhich has already been initiated. In order to initiate the process, itis necessary to supply to the system from an outside source, sufficientof the defined light aliphatics to first saturate the extract or thereflux loop and then a sufficient amount to bring the defined lightaliphatics up to the prescribed level. This is also done for the solventand water. Once the process is initiated, these components are recycledwith makeup being added when necessary.

Subject process is found to be particularly advantageous for feedstockcontaining at least about 80 percent by weight aromatics and even moreso for those containing about 90 percent or more aromatics. Optimumperformance is achieved when the benzene content is also high, i.e., atleast about 25 percent by weight of the feedstock. The process is foundto be beneficial for feedstocks having an aromatics content of at leastabout 40 percent by weight, however.

What is claimed is:

1. In a continuous solvent extraction distillation process for therecovery of aromatic hydrocarbons having boiling points in the range ofabout 80C. to about 175C. from a feedstock containing aliphatichydrocarbons and at least about 40 percent by weight, based on theweight of the feedstock, of said aromatic hydrocarbons wherein thefeedstock contains no more than about 4 percent by weight, based on theweight of the feedstock, of light aliphatics consisting essentially ofaliphatic and cycloaliphatic hydrocarbons, each having no more than 5carbon atoms, a boiling point no higher than about 50C. and beingcondensable at a pressure of no more than about 3 atmospheres and at atemperature of no less than about 50C.; a single extraction columnhaving three to 25 theoretical stages is used for the solvent extractionto provide an extract; and a single distillation column is used todistill the extract to provide a mixture of the aromatic hydrocarbonscomprising the following steps:

a. introducing the feedstock into the extraction column at the middletheoretical stage thereof;

b. contacting the feedstock in the extraction column with a mixture ofwater and a solvent, said solvent being a water-miscible organic liquidhaving a boiling point of at least about 200C. and having adecomposition temperature of at least about 225C., and with refluxhydrocarbons introduced into the extraction column below the bottomtheoretical stage thereof to provide the extract comprising aromatichydrocarbons, reflux aliphatic hydrocarbons, solvent, and water and araffinate comprising essentially aliphatic hydrocarbons;

c. introducing the extract into the distillation column to separate thearomatic hydrocarbons and the reflux hydrocarbons from the extract;

d. recycling the reflux hydrocarbons from step (c) to the extractioncolumn as provided in step (b); and

e. recovering the aromatic hydrocarbons of step (c);

the improvement comprising introducing a sufficient amount of the lightaliphatics defined above into the extraction column at the middletheoretical stage thereof to provide a total percent by weight of saidlight aliphatics, based on the weight of the feedstock, in the range ofabout 5 percent to about 12 percent.

2. The process of claim 1 comprising the following additional steps:

f. introducing the rafflnate formed in step (b) into a raffinatedistillation zone to separate the light aliphatics defined in claim 1therefrom;

g. recycling the defined light aliphatics from step (f) to the middletheoretical stage of the extraction column; and

h. recovering the balance of the raffinate from step 3. The process ofclaim 2 wherein the feedstock and the defined light aliphatics enter theextraction column at about the same point.

4. The process of claim 3 wherein the temperature in the extractioncolumn is in the range of about C. to about 200C, the pressure in theextraction column is in the range of about 75 psig to about 200 psig,the temperature in the distillation column is in the range of about C.to about 200C, and the pressure in the distillation column is in therange of about 10 psig to about 35 psig. I

5. The process of claim 4 wherein the feedstock contains at least about80 percent by weight, based on the weight of the feedstock, of aromatichydrocarbons.

6. The process of claim 4 wherein i. the ratio of solvent to feedstockin the extraction column is in the range of about 3 to about 12 parts byweight of solvent to one part by weight of feedstock;

ii. the amount of water in the extraction column is about 1 percent toabout 8 percent by weight based on the weight of the solvent in saidcolumn;

iii. the ratio of reflux to feedstock in the extraction column is in therange of about 0.5 to about 1.5 parts by weight of reflux to one part byweight of feedstock; and

iv. the ratio of water to aromatic hydrocarbons in the distillationcolumn is in the range of about 0.1 to about 0.5 part by weight of waterto one part by weight of aromatic hydrocarbons in said column.

7. The process of claim 6 wherein the solvent is a polyalkylene glycol.

8. The process of claim 7 wherein the solvent is tetraethylene glycol.

9. The process of claim 8 wherein the feedstock contains at least about80 percent by weight, based on the weight of the feedstock, of aromatichydrocarbons.

10. The process of claim 8 wherein the extraction column has five to 12theoretical stages.

11. In a continuous solvent extraction-distillation process for therecovery of aromatic hydrocarbons having boiling points in the range ofabout 80C. to about C. from a feedstock containing aliphatichydrocarbons and at least about 40 percent by weight, based on theweight of the feedstock, of said aromatic hydrocarbons wherein thefeedstock contains no more than about 4 percent by weight, based on theweight of the feedstock, of light aliphatics consisting essentially ofaliphatic and cycloaliphatic hydrocarbons, each having no more than fivecarbon atoms, a boiling point no higher than about 50C. and beingcondensable at a pressure of no more than about 3 atmospheres and at atemperature of no less than about 50C.; a single extraction columnhaving three to 25 theoretical stages is used for the solvent extractionto provide an extract;

and a single distillation column is used to distill the extract toprovide a mixture of the aromatic hydrocarbons comprising the followingsteps:

a. introducing the feedstock into the extraction co1- umn at the middletheoretical stage thereof;

b. contacting the feedstock in the extraction column with a mixture ofwater and a solvent, said solvent being a water-miscible organic liquidhaving a boiling point of at least about 200C. and having adecomposition temperature of at least about 225C., and with refluxhydrocarbons introduced into the extraction column below the bottomtheoretical stage thereof to provide the extract comprising aromatichydrocarbons, reflux aliphatic hydrocarbons, solvent, and water and araffinate comprising essentially aliphatic hydrocarbons;

c. contacting the extract with steam in the distillation column toprovide an overhead distillate comprising a reflux hydrocarbons phaseand a water phase, a side cut distillate comprising an aromatichydrocarbons phase and a water phase, and bottoms comprising a mixtureof solvent and water;

d. dividing the water phase of the overhead distillate into first andsecond streams;

e. contacting the raffinate with the first stream to provide analiphatic hydrocarbons phase and a water phase;

. contacting the second stream with an aromatic hydrocarbons streamcontaining at least 95 percent aromatic hydrocarbons, the amount of saidstream being in the range of about 0.1 percent to about 5 percent byweight of the total aromatic hydrocarbons in the feedstock, to form anaromatic hydrocarbons phase and a water phase;

g. contacting the aromatic hydrocarbons phase of the side-cut distillatewith the water phase of (f) to form an aromatic hydrocarbons phase and awater phase;

h. contacting the water phase of step (e) with an aromatic hydrocarbonsstream containing at least 95 percent aromatic hydrocarbons, the amountof said stream being in the range of about 0.1 percent to about 5percent by weight of the total aromatic hydrocarbons in the feedstock,to form an aromatic hydrocarbons phase and a water phase;

. recycling the water phases of steps (g) and (h) to the distillationzone where said water phases are essentially converted to steam;

j. recycling the reflux hydrocarbons phase of the overhead distillateand the bottoms of step (c) to the extraction zone to provide refluxhydrocarbons and mixture of water and solvent, respectively, for step(b); and

k. recovering the aromatic hydrocarbons phase of step' (g) and thealiphatic hydrocarbons phase of p the improvement comprising introducinga sufficient amount of the light aliphatics defined above into theextraction column at the middle theoretical stage thereof to provide atotal percent by weight of said light aliphatics, based on the weight ofthe feedstock, in the range of about 5 percent to about 12 percent.

12. In a continuous solvent extraction-distillation process for therecovery of aromatic hydrocarbons having boiling points in the range ofabout 80C. to about 175C. from a feed-stock containing aliphatichydrocarbonsand at least about 40 percent by weight, based on the weightof the feedstock, of said aromatic hydrocarbons wherein the feedstockcontains no more than about 4 percent by weight, based on the weight ofthe feedstock, of light aliphatics consisting essentially of aliphaticand cycloaliphatic hydrocarbons, each having no more than five carbonatoms, a boiling point no higher than about 50C. and being condensableat a pressure of no more than about 3 atmospheres and at a temperatureof no less than about 50C.; a single extraction column having three to25 theoretical stages is used for the solvent extraction to provide anextract; and a single distillation column is used to distill the extractto provide a mixture of the aromatic hydrocarbons comprising thefollowing steps:

a. introducing the feedstock into the extraction column at the middletheoretical stage thereof;

b. contacting the feedstock in the extraction column with a mixture ofwater and a solvent, said solvent being a water-miscible organic liquidhaving a boiling point of at least about 200C. and having adecomposition temperature of at least about 225C., and with refluxhydrocarbons introduced into the extraction column below the bottomtheoretical stage thereof to provide the extract comprising aromatichydrocarbons, reflux aliphatic hydrocarbons, solvent, and water and araffinate comprising essentially aliphatic hydrocarbons;

c. contacting the extract with steam in the distillation column toprovide an overhead distillate comprising a reflux hydrocarbons phaseand a water phase, a side cut distillate comprising an aromatichydrocarbons phase and a water phase, and bottoms comprising a mixtureof solvent and water;

. contacting the raffinate with the water phase of the overheaddistillate to provide an aliphatic hydrocarbons phase and a water phase;

e. contacting the water phase of step (d) with an aromatic hydrocarbonsstream containing at least percent aromatic hydrocarbons, the amount ofsaid stream being in the range of about 0.1 percent to about 5 percentby weight of the total aromatic hydrocarbons in the feedstock, to forman aromatic hydrocarbons phase and a water phase;

f. contacting the aromatic hydrocarbons phase of the side-cut distillatewith the water phase of (e) to form an aromatic hydrocarbons phase and awater phase;

g. recycling the water phase of step (f) to the distillate zone wheresaid water phase is essentially converted to steam;

h. recycling the reflux hydrocarbons phase of the overhead distillateand the bottoms of step (c) to the extraction zone to provide refluxhydrocarbons and mixture of water and solvent, respectively, for step(b); and

. recovering the aromatic hydrocarbons phase of step (f) and thealiphatic hydrocarbons phase of p the improvement comprising introducinga sufficient amount of the light aliphatics defined above into theextraction column at the middle theoretical stage thereof to provide atotal percent by weight of said light aliphatics, based on the weight ofthe feedstock, in the range of about 5 percent to about 12 percent.

13. The process of claim 11 wherein the solvent is a polyalkyleneglycol. polyalkylene glycol' 16. The process of claim 15 wherein thesolvent is 14. The process of claim 13 wherein the solvent 18tetraethylene glycol.

15. The process of claim 12 wherein the solvent is a 5 tetraethyleneglycol.

2. The process of claim 1 comprising the following additional steps: f.introducing the raffinate formed in step (b) into a raffinatedistillation zone to separate the light aliphatics defined in claim 1therefrom; g. recycling the defined light aliphatics from step (f) tothe middle theoretical stage of the extraction column; and h. recoveringthe balance of the raffinate from step (f).
 3. The process of claim 2wherein the feedstock and the defined light aliphatics enter theextraction column at about the same point.
 4. The process of claim 3wherein the temperature in the extraction column is in the range ofabout 100*C. to about 200*C., the pressure in the extraction column isin the range of about 75 psig to about 200 psig, the temperature in thedistillation column is in the range of about 135*C. to about 200*C., andthe pressure in the distillation column is in the range of about 10 psigto about 35 psig.
 5. The process of claim 4 wherein the feedstockcontains at least about 80 percent by weight, based on the weight of thefeedstock, of aromatic hydrocarbons.
 6. The process of claim 4 whereini. the ratio of solvent to feedstock in the extraction column is in therange of about 3 to about 12 parts by weight of solvent to one part byweight of feedstock; ii. the amount of water in the extraction column isabout 1 percent to about 8 percent by weight based on the weight of thesolvent in said column; iii. the ratio of reflux to feedstock in theextraction column is in the range of about 0.5 to about 1.5 parts byweight of reflux to one part by weight of feedstock; and iv. the ratioof water to aromatic hydrocarbons in the distillation column is in therange of about 0.1 to about 0.5 part by weight of water to one part byweight of aromatic hydrocarbons in said column.
 7. The process of claim6 wherein the solvent is a polyalkylene glycol.
 8. The process of claim7 wherein the solvent is tetraethylene glycol.
 9. The process of claim 8wherein the feedstock contains at least about 80 percent by weight,based on the weight of the feedstock, of aromatic hydrocarbons.
 10. Theprocess of claim 8 wherein the extraction column has five to 12theoretical stages.
 11. In a continuous solvent extraction-distillationprocess for the recovery of aromatic hydrocarbons having boiling pointsin the range of about 80*C. to about 175*C. from a feedstock containingaliphatic hydrocarbons and at least about 40 percent by weight, based onthe weight of the feedstock, of said aromatic hydrocarbons wherein thefeedstock contains no more than about 4 percent by weight, based on theweight of the feedstock, of light aliphatics consisting essentially ofaliphatic and cycloaliphatic hydrocarbons, each having no more than fivecarbon atoms, a boiling point no higher than about 50*C. and beingcondensable at a pressure of no more than about 3 atmospheres and at atemperature of no less than about 50*C.; a single extraction columnhaving three to 25 theoretical stages is used for the solvent extractionto provide an extract; and a single distillation column is used todistill the extract to provide a mixture of the aromatic hydrocarbonscomprising the following steps: a. introducing the feedstock into theextraction column at the middle theoretical stage thereof; b. contactingthe feedstock in the extraction column with a mixture of water and asolvent, said solvent being a water-miscible organic liquid having aboiling point of at least about 200*C. and having a decompositiontemperature of at least about 225*C., and with reflux hydrocarbonsintroduced into the extraction column below the bottom theoretical stagethereof to provide the extract comprising aromatic hydrocarbons, refluxaliphatic hydrocarbons, solvent, and water and a raffinate comprisingessentially aliphatic hydrocarbons; c. contacting the extract with steamin the distillation column to provide an overhead distillate comprisinga reflux hydrocarbons phase and a water phase, a side cut distillatecomprising an aromatic hydrocarbons phase and a water phase, and bottomscomprising a mixture of solvent and water; d. dividing the water phaseof the overhead distillate into first and second streams; e. contactingthe raffinate with the first stream to provide an aliphatic hydrocarbonsphase and a water phase; f. contacting the second stream with anaromatic hydrocarbons stream containing at least 95 percent aromatichydrocarbons, the amount of said stream being in the range of about 0.1percent to about 5 percent by weight of the total aromatic hydrocarbonsin the feedstock, to form an aromatic hydrocarbons phase and a waterphase; g. contacting the aromatic hydrocarbons phase of the side-cutdistillate with the water phase of (f) to form an aromatic hydrocarbonsphase and a water phase; h. contacting the water phase of step (e) withan aromatic hydrocarbons stream containing at least 95 percent aromatichydrocarbons, the amount of said stream being in the range of about 0.1percent to about 5 percent by weight of the total aromatic hydrocarbonsin the feedstock, to form an aromatic hydrocarbons phase and a waterphase; i. recycling the water phases of steps (g) and (h) to thedistillation zone where said water phases are essentially converted tosteam; j. recycling the reflux hydrocarbons phase of the overheaddistillate and the bottoms of step (c) to the extraction zone to providereflux hydrocarbons and mixture of water and solvent, respectively, forstep (b); and k. recovering the aromatic hydrocarbons phase of step (g)and the aliphatic hydrocarbons phase of step (e); the improvementcomprising introducing a sufficient amount of the light aliphaticsdefined above into the extraction column at the middle theoretical stagethereof to provide a total percent by weight of said light aliphatics,based on the weight of the feedstock, in the range of about 5 percent toabout 12 percent.
 12. In a continuous solvent extraction-distillationprocess for the recovery of aromatic hydrocarbons having boiling pointsin the range of about 80*C. to about 175*C. from a feed-stock containingaliphatic hydrocarbons and at least about 40 percent by weight, based onthe weight of the feedstock, of said aromatic hydrocarbons wherein thefeedstock contains no more than about 4 percent by weight, based on theweight of the feedstock, of light aliphatics consisting essentially ofaliphatic and cycloaliphatic hydrocarbons, each having No more than fivecarbon atoms, a boiling point no higher than about 50*C. and beingcondensable at a pressure of no more than about 3 atmospheres and at atemperature of no less than about 50*C.; a single extraction columnhaving three to 25 theoretical stages is used for the solvent extractionto provide an extract; and a single distillation column is used todistill the extract to provide a mixture of the aromatic hydrocarbonscomprising the following steps: a. introducing the feedstock into theextraction column at the middle theoretical stage thereof; b. contactingthe feedstock in the extraction column with a mixture of water and asolvent, said solvent being a water-miscible organic liquid having aboiling point of at least about 200*C. and having a decompositiontemperature of at least about 225*C., and with reflux hydrocarbonsintroduced into the extraction column below the bottom theoretical stagethereof to provide the extract comprising aromatic hydrocarbons, refluxaliphatic hydrocarbons, solvent, and water and a raffinate comprisingessentially aliphatic hydrocarbons; c. contacting the extract with steamin the distillation column to provide an overhead distillate comprisinga reflux hydrocarbons phase and a water phase, a side cut distillatecomprising an aromatic hydrocarbons phase and a water phase, and bottomscomprising a mixture of solvent and water; d. contacting the raffinatewith the water phase of the overhead distillate to provide an aliphatichydrocarbons phase and a water phase; e. contacting the water phase ofstep (d) with an aromatic hydrocarbons stream containing at least 95percent aromatic hydrocarbons, the amount of said stream being in therange of about 0.1 percent to about 5 percent by weight of the totalaromatic hydrocarbons in the feedstock, to form an aromatic hydrocarbonsphase and a water phase; f. contacting the aromatic hydrocarbons phaseof the side-cut distillate with the water phase of (e) to form anaromatic hydrocarbons phase and a water phase; g. recycling the waterphase of step (f) to the distillate zone where said water phase isessentially converted to steam; h. recycling the reflux hydrocarbonsphase of the overhead distillate and the bottoms of step (c) to theextraction zone to provide reflux hydrocarbons and mixture of water andsolvent, respectively, for step (b); and i. recovering the aromatichydrocarbons phase of step (f) and the aliphatic hydrocarbons phase ofstep (d); the improvement comprising introducing a sufficient amount ofthe light aliphatics defined above into the extraction column at themiddle theoretical stage thereof to provide a total percent by weight ofsaid light aliphatics, based on the weight of the feedstock, in therange of about 5 percent to about 12 percent.
 13. The process of claim11 wherein the solvent is a polyalkylene glycol.
 14. The process ofclaim 13 wherein the solvent is tetraethylene glycol.
 15. The process ofclaim 12 wherein the solvent is a polyalkylene glycol.
 16. The processof claim 15 wherein the solvent is tetraethylene glycol.